He was born in then L'viv, Poland as Karl Sobelsohn. A member of the RSDLP (Russian Social Democratic Labour Party) since 1898, he participated in the 1905 revolution in Warsaw. He took an anti-war stand during WWI while living in Switzerland, and supported the Bolsheviks and joined the party in 1917 after the October Revolution. He was in Germany in 1918-20 organising the German Communist movement. He returned to Russia and joined Comintern but his influence decreased and he lost his place on the Central Committee in 1924 and was expelled from the party in 1927. Radek was re-admitted to the party in 1930, but in the Stalinist purges of the 1930s he was accused of treason and confessed at the Trial of the Seventeen (1937, also called the Second Moscow Trial). He died in prison.